drivers: modem: quectel-bg9x: mdm unsol ready

Using bg9x driver with bg95 specifically has the issue that BG95
modem replies with "RDY" when it's not fully initialized and AT
commands sent to the modem after this point would result in modem
reply of "ERROR". This commit adds a new prj.conf field in the build
configuration for custom unsolicited app ready string, with
the default being BG96 one ("RDY").

Signed-off-by: Andrei Hutanu <andrei.hutanu.i@gmail.com>